You can use your editing program to create a 512*512 area that is transparent then cut what you want from the old poster and paste onto the new area and make that into a .vtf as transparency makes everything else that isn't used to be transparent when placed.
Gimp is an ideal free program as you can create the transparency pretty easy.

You can use your editing program to create a 512*512 area that is transparent then cut what you want from the old poster and paste onto the new area and make that into a .vtf as transparency makes everything else that isn't used to be transparent when placed.
Gimp is an ideal free program as you can create the transparency pretty easy.